The present invention relates to a developer supporting member for developing a static latent image on a photosensitive drum, a developing roller using the developer supporting member, a developing device, and an image forming apparatus.
In a conventional image forming apparatus of an electro-photography type, a developing roller and a toner supply roller charge toner through friction, so that toner adheres to a circumferential surface of the developing roller. Then, the developing roller transports toner to a photosensitive drum as an image supporting member. Accordingly, toner adheres to a static latent image on a circumferential surface of the photosensitive drum, so that the static latent image is visualized.
In the conventional image forming apparatus, the developing roller is formed of a conductive shaft and a semi-conductive elastic layer disposed on a circumferential surface of the conductive shaft. The elastic layer is formed of a rubber material such as a urethane rubber, an NBR, an EPDM, a silicone rubber and the like containing an ion conductive agent or an electron conductive agent such as carbon, a conductive filler, and the like.
In the conventional image forming apparatus, a surface of the developing roller is treated with a charging agent, a surface modification agent, and the like, so that toner is easily charged or the photosensitive drum is not stained through a chemical reaction. The surface treatment uses a material with small compressive permanent strain, that is, a material in which a nip imprint is difficult to form under a state pressed against the photosensitive drum for a long period of time.
Patent Reference has disclosed a technology of forming a surface treated layer with small compressive permanent strain. In the technology, a surface of an elastic layer formed of a urethane rubber with high ion conductivity is treated with an isocyanate solution.    Patent Reference: Japanese Patent Publication No. 2005-148470
In the conventional image forming apparatus, a developing blade is provided to slide against the circumferential surface of the developing roller. Further, a sealing member is provided to slide against the circumferential surface of the developing roller for preventing toner from scattering outside a casing of a fixing device. Accordingly, the circumferential surface of the developing roller is divided into two portions facing outside and inside of the casing with the developing blade and the sealing member.
In the conventional image forming apparatus, when the developing device starts operating, a surface potential of the developing roller increases. When the developing device stops operating, the surface potential of the developing roller gradually decreases due to external moisture. In this case, the portion facing inside of the casing contacts with toner over an area larger than that of the portion facing outside of the casing. Accordingly, in the portion facing inside of the casing, the surface potential of the developing roller decreases more gradually due to external moisture. As a result, when the developing device stops operating for a long period of time, a difference may be generated in the surface potential of the developing roller, so that a density variance may occur in an image. In other words, when the developing device stops operating for a long period of time, a density variance in a bad shape (refer to an idle lateral band) may be generated with an interval corresponding to a circumferential length of the developing roller when the developing device resumes operating.
In view of the problems described above, an object of the present invention is to provide a developer supporting member capable of preventing the idle lateral band. Another object of the present invention is to provide a developing roller using the developer supporting member, a developing device, and an image forming apparatus.
Further objects of the invention will be apparent from the following description of the invention.